1. Field of the Invention
The present invention relates to a mobile radio terminal capable of roaming domestically or overseas.
2. Description of the Related Art
In a cellular mobile radio terminal (for example, cf. 3GPP STANDARDS TS25.331, TS25.304, TS24.008), neighbor cells are searched at a standby time to maintain preferable receiving quality at any time. If, as a result of the search, a cell of preferable receiving quality is detected and it is discriminated on the basis of predetermined discrimination standards that a serving cell needs to be changed, the mobile radio terminal receives information from the cell and discriminates the necessity of location registration from a network number, location registration area number, and the like included in the received information.
If the mobile radio terminal discriminates that the location registration needs to be executed, the mobile radio terminal executes a location registration process.
At this time, in response to the location registration request transmitted to the network, a cause value indicating a location registration rejection such as roaming prohibition is often transmitted from the network to reject the location registration. In this case, the mobile radio terminal recognizes that the location registration area where the location registration request has been made is an area of roaming prohibition, from the notification, and stores the location registration area number as a roaming prohibition area.
After that, when the mobile radio terminal receives the information of the neighbor cell, the mobile radio terminal collates the location registration area number notified by the information with the location registration area number of the roaming prohibition area stored in the terminal. If the location registration area numbers match, the mobile radio terminal recognizes that the location registration area is the area of roaming prohibition, and operates not to execute the location registration.
In the mobile radio terminal of the cellular system, the cell is reselected in accordance with movement, in the above steps. However, if the condition that the receiving quality of the roaming prohibition cell is high is maintained as a result of search of the neighbor cell, the process of receiving the information from the roaming prohibition cell is repeated and the current consumption is therefore increased.
To discriminate whether the cell detected by the search prohibits roaming, it is necessary to receive the information and discriminate whether the network number and the location registration area number in the information match those of the roaming prohibition cell stored in the mobile radio terminal, as described above. For this reason, the process of receiving the information is required, which results in the above problem.
To solve this problem, a method of using the frequency of the prohibition cell as a prohibition frequency, excluding the prohibition frequency from the search and not executing the process of receiving the information of the prohibition cell has been proposed (cf., for example, Jpn. Pat. Appln. KOKAI Publication No. 2006-311329).
Since reception in a specified frequency is prohibited in this method, no problem occurs in a case of prohibiting roaming in unit of network. However, if roaming is prohibited in unit of location registration area, in the networks of the same frequency, the location registration area where roaming is not prohibited is regarded as the roaming prohibition area and roaming cannot be executed.